Radial flow reactor

ABSTRACT

In apparatus for coating a radial flow of reactant gases is established over the substrates. A radio frequency radial flow cylindrical reactor is utilized to effect low temperature plasma deposition. Electrodes for generating the RF glow discharge are disposed within the chamber of the reactor. An annular electrode forms the support for the substrates which are disposed around the circumference of concentric circles on a surface of the support. The center region of the support communicates with an exhaust port extending downwardly therefrom. Gas flow is established over the outer edge of the support and flows in a laminar manner radially over the substrates to be exhausted through the exhaust port.

United States Patent Reinberg 1 Sept. 1 l, 1973 1 RADIAL FLOW REACTOR 3,424,629 1/1969 Ernst 8:31. 118/495 x 3,594,227 7/1971 Oswald t 1 117/107.1 [75] Inventor Remberg Dallas 3,641,974 2/1972 Yamada et a1... 118/48 7 Assigneez Texas Instruments Incorporated, 3,696,779 10/1972 Murai et al. 118/48 Dallas, Tex. Primary Examiner-Morris Kaplan 1 Flled: 1 1971 At10rney-Harold Levine et al.

[21] Appl. No.: 192,957

[57] ABSTRACT In apparatus for coating a radial flow of reactant gases [g2] U.S. C1. is established Over the Substrates A radio frequency ra; M is h 1 4 dial flow cylindrical reactor is utilized to effect low I 1 1 1 g 1 temperature plasma deposition. Electrodes for generatl I 5 i ing the RF glow discharge are disposed within the 9 I 9 G chamber of the reactor. An annular electrode forms the support for the substrates which are disposed around [56] References cued the circumference of concentric circles on a surface of UNITED STATES PATENTS the support. The center region of the support commu- 2,580,131 1 12/1951 Rowell 118/52X nicates with an exhaust port extending downwardly 3,057,690 10/1962 1 Ch et a] 95 UX therefrom. Gas flow is established over'the outer edge Sm1th t X of the upport and flows in a laminar manner radially 3,323,436 1/1967 Hafer et a] 95/89 G Over the substrates to be exhausted through the exhaust 3,329,601 7/1967 Mattox 1l8/49.5 X on 3,364,833 1/1968 Mulvany... 95/89 G p 3,408,982 11/ 1968 9 Claims, 3 Drawing Figures C apita 1 18/495 SOURCE PATENIEU 1 I973 ME! 1 OF 2 20\ RF M souRcE x 80 a0 36 a0 f 24 25 f u \K 4 f*\\\ my ///1 w v///A w M11112 V/l/ l w 28 44/ 30 80m! HT 22 26 /|I H: A

i I :-\,32 ,2 fif 5 42 I 1 m JIM III l l F/g,/ 34 I 38 INPUT GASES VACUUM VACUUM PRESSURE MP GAUGE LEAK TOGGLE vALvE vALvE 54 58 O 56 NEEDLE TOGGLE N2 FLOW METER vALvE vALvE TOGGLE MIXER I REACTOR vALvE VACUUM PRESSURE GAUGE FLOW METER LEAK TOGGLE VALVE VALVE Fig. 3

RADIAL FLOW REACTOR This invention pertains generally to apparatus for depositing coatings on suitable substrates and more particularly to a radial flow reactor for coating substrates utilizing laminar flow of reactant gases in a radial direction over a surface.

The stability of semiconductor devices, particularly metal-insulator-semiconductors, largely depends on the manner in which they are passivated and how the completed devices are isolated from the environment. To date many different materials have been tried as passivators including deposited silane oxides (both doped and undoped) aluminum oxide, doped glasses of varying compositions, and more recently silicon nitride. Withthe exception of silicon nitride, all of these materials can be deposited at relatively low temperatures by an appropriate chemical or electro-chemical process. For nitrides, standard chemical vapor deposition techniques are typically carried out at about 750 to 800 C using a gas mixture of silane, ammonia, and hydrogen.

The process for depositing nitride at low temperatures in a glow discharge has recently been under investigation. It has been found that the controlled dissociation and recombination of gasous mixtures in a glow discharge is a useful method for depositing polymerized thin films. In glow discharge polymerization it has been found that all surfaces in the vicinity of the glow become covered with a thin polymer film. Films of oxides, nitrides or carbides of metals or semi-metals may be formed by the controlled dissociation and selective combination of volatile metal or semi-metal containing compounds in a low pressure glow discharge.

Glow discharge deposition is also commonly referred to as plasma deposition. Plasma is defined as a state within agas in which there are substantially equal number of positively and negatively charged particles, the positive particles being ions, either atomic or molecular and the negative particles being electrons. The plasma may be established by a variety of methods but it is preferred to apply an electric field to establish the plasma utilizing a voltage which alternates at a radio frequency.

Utilizing low temperature plasma deposition, it is possible to form coatings on a substrate of nitride, oxides, oxinitrides, carbides and amorphous silicon. These coatings may be used in a variety of applications, such as, by the way of example, for gate dielectrics, multilevel interconnect systems, scratch resistant coatings, thin and thick film, light guides, etch masks, and passivation.

A major advantage of the RF plasma deposition process is that it replaces the thermal energy normally used to activate the chemical process governing the deposition of a film by the electrical energy in the gas discharge. This enables deposition of highly stable coatings at relatively low temperatures, generally in the range of 200 C.

To date, conventional RF glow systems typically are arranged sothat the electrodes which excite the discharge are external to the vacuum chamber in which the reaction occurs, presumably enabling operation Without contamination from the electrodes. Such a structure generally requires higher operating powers than are desirable and contributes to lack of uniformity in the deposited coating.

Additionally, conventional RF glow discharge reactors are of the linear type. One such reactor consists of a straight tube section in which thematcrial is arranged so that the surface to be coated faces the center of the tube. The tube must be fitted at one end with an appropriately tapered section that can be attached to a vacuum system and the other end of the tube has a sealable cap through which the samples are put into the active zone. Both 0 ring and flat gasket closures have typi cally been utilized. An appropriate holder supports the material to be coated near the center of the tube.

A major problem associated with linear reactors per tains to the limited production capability that is obtainable. At this point it is noted that in coating, for example, a slice of semiconductor material, it is very important that the coating be uniform across the surface of the slice. A typical slice may, for example, be 2 inches in diameter. in the linear reactor the uniformity of deposition varies substantially as a function of distance along the tube. The non-uniformity is believed to be due to depletion of available reactant gas (such as, e.g., silane) along the tube length. In addition, wall affects introduce a nonuniformity across the tube diameter. The relatively low pressure necessary for sufficiently uniform deposition along the tube axis imposes an additional limiting effect on deposition rate since this is a function of pressure, increasing approximately linearly. Deposition rates also increase with increased RF power. At high pressures and RF powers, it is difficult to achieve uniformity except over a relatively short length of the reaction zone, typically about 4-5 inches. To date glow discharge reactors have generally been used only on an experimental basis and are capable of handling 1 or 2 two-inch slices. In a production environment, however, it would be desirable to be able to process a significantly increased number of slices simultaneously Accordingly, a primary object of the present invention is to provide an improved reactor for simulta neously applying uniform coatings to a plurality of suitable substrates. Another object of the present invention is to provide a RF glow discharge reactor having increased production capability.

A further object of the invention is to provide a radio frequency radial flow cylinderical glow discharge reactor.

An additional object of the present invention is to provide an RF glow discharge reactor in which the electrodes are positioned within the vacuum chamber.

Still another object of the invention is to provide an RF glow discharge reactor having increased deposition rate capability.

Yet another object is to provide an improved method of coating a plurality of substrates by flowing a laminar stream of reactant gases over the substrates in a radial direction.

Briefly and in accordance with one aspect of the present invention, applicant has provided an RF radial flow cylindrical ractor for coating a plurality of substrates with an inorganic material by low temperature plasma deposition techniques. The reactor includes an evacuable chamber, a support having an outer region for holding a plurality of substrates and a central region defining an aperture, means for generating a radio frequency glow discharge within the evacuable chamber adjacent the substrates, means for establishing a radial flow of reactant gases suitable for forming the desired coating which reactant gases flow from the outer region of the support to the inner or central region of the support in a radial flow pattern, and means in communication with the central region of the support for exhausting the gases. Suitable means are also provided for heating the substrates to a preselected temperature such as on the order of 200 to 300 C. Further the means for generating a glow discharge preferably include substantially parallel electrodes within the evacuable chamber. In this regard it is noted it may be desirable to form the electrodes to have a slight curvature to enhance uniformity of deposition.

The invention also provides an apparatus for effecting plasma deposition of a coating which includes the steps of evacuating the chamber, positioning within the chamber a support having a central exhaust region communicating with the exterior of the chamber and an outer region exposed for holding a plurality of substrates to be coated, generating a radio frequency glow discharge in the region of the substrate, radially flowing reactant gases over the outer edge of the support toward the central portion of the support and exhausting the gases through the central exhaust region.

Other novel features, objects and advantages of the present invention will become apparent upon reading the following detailed description of illustrative embodiments of the invention in conjunction with the drawings wherein:

FIG. 1 is a cross-sectional view of the RF radial flow reactor in accordance with one embodiment of the present invention;

FIG. 2 is a plan view illustrating an embodiment of the present invention wherein one electrode is disposed for receiving 34 discrete semiconductor slices for deposition; and

FIG. 3 is a flow chart illustrating the flow of gases to the reaction area of the cylindrical reactor illustrated in FIG. 1.

The radial flow cylindrical reactor of the present invention is applicable to a varity of applications. By way of example, the reactor may advantageously be utilized for epitaxial deposition by standard thermal techniques. The laminar flow of reactant gases over the substrates enables formation of uniform epitaxial layers (doped or undoped) on a plurality of suitable substrates in one operation. Also the reactor is advantageous for sputtering techniques, particularly reactive sputtering. The cylindrical reactor is particularly advantageous for low temperature plasma deposition, and the present invention will be described hereinafter in an illustrative embodiment designed for this type of deposition. It is understood that this embodiment is, by way of example only, and is not intended to limit the invention. It will be appreciated that other materials, reactant gases, etc., known to those skilled in the art will be utilized for optimization of epitaxial and sputtering embodiments.

With reference to FIG. 1, a cross-section of the cylindrical radial flow radio frequency illustrating the preferred embodiment of the present invention is shown generally at 10. The reactor includes a bottom plate 12, and a top plate 14 which may, by way of example, comprise standard 18 inch stainless steel bell jar plates. Side wall 16, typically an open-ended glass or quartz bell jar, is connected to the top and bottom plates 12 and 14 in sealing relationship to define an evacuable chamber 18. All the components used may, by way of example, comprise stainless steel welded construction using standard diameters of tubing, plate, sheet, etc. Dimensions are not critical and may vary by as much as 20 per cent without significantly affecting operation of the reactor.

The top plate 14 comprises one electrode required for establishing the RF glow discharge and is electrically connected to a source of RF energy illustrated generally at 20. The RF source can readily be supplied by a small HAM transmitter or RF generator such as is found in any of the ASHERS commonly used in semiconductor processing. Typically only several watts of RF power are required. It is convenient to have a matching network situated near the electrodes to help in getting the power to the system with a minimum of reflection. Although reflections themselves do not seriously affect the process, they may cause damage to the generator. HAM transmitters with pi network outputs are quite satisfactory and a watt transmitter is considered more than adequate.

The second electrode of the RF plasma deposition system is illustrated at 22. Preferably this electrode is circular in structure and the top surface 24 is designed for receiving a plurality of semiconductor slices 26 or for receiving a support plate upon which the slices 26 are mounted. The outer edge 28 of the electrode 22 is spaced from the side wall 16. The electrode 22 is centered so that the outer edge 28 is evenly spaced from the side wall 16 of the chamber 18. Further, the bottom surface 30 of the electrode 22 is spaced from the bottom 12 of the chamber 18. A sheath or tube 32 communicates with the interior of the chamber 18 extending through the bottom plate 12 of the chamber in sealing relationship. Preferably the sheath 32 terminates at the interior side of the plate 12. A second smaller sheath 34 is substantially centered within the sheath 32 and extends coaxially therethrough. The sheath 34 extends into the interior of the chamber 18 and contacts the electrode 22. The electrode 22 has a center region illustrated generally at 36 which defines an aperture therethrough. The sheath 34 extends into this aperture in sealing relationship and terminates at the surface 24 of the electrode 22. The other end of the sheath 34 is connected to a vacuum system for evacuating the interior of the chamber 18. The sheath 32 is sealed at end 38 to define adistribution manifold or chamber between the exterior wall of the sheath 34 and the interior wall of the sheath 32. This mixing chamber is shown generallyat 40. A tube 42 is formed in communication with the interior of the mixing chamber 40. The input to the tube 42 comprises the reactant gases required to coat the substrate with the desired coating.

Means for heating the semiconductor slices 26 are shown generally as stripline heaters 44 adjacent the bottom surface 30 of the electrode 22. Any suitable heating such as lamps, etc., may be utilized to heat the semiconductor slices 26 to the required 200 to 300 C range.

In order to obtain uniform deposition the gas velocity flowing through the chamber 18 must be sufficiently high. For the reaction chamber illustrated in FIG. 1 a pump having a capacity on the order of c.f. per minute is adequate. Various vacuum pumps suitable for providing such a flow rate are commercially available, such as, e.g., a pump available from Leybold-Heraeus, Model W.S.250.

In operation, the reactor is first loaded with slices 26 of material to be coated. By way of example, the slices may be loaded on a support 46 such as illustrated in FIG. 2 which support is then secured to the surface 24 of the electrode 22. The support 46, illustrated in FIG. 2, is capable of holding 34 discrete two-inch slices of material. After loading the slices, evacuation of the chamber 18 is commenced. Since most depositions are effected at temperature between 200 and 300 C, the heating means are generally initiated during evacuation. Both before and during the pump down, the flow of inert gas is continuously open to the reactor, preventing contamination from backstreaming. If a predeposition cleanup is desired, for example, in a nitrogen discharge, it is convenient to perform this cleanup during the warmup period. The RF power 20 is turned on and allowed to operate until the samples or slices 26 reach the desired temperature. When a lamp-heating system is utilized, it may, by way of example, take on the order of minutes to reach 200 C from room temperature. Before the actual deposition is started, the RF power is turned off, the reactive gases turned on through the tube 42 (silane and nitrogen for nitride deposition and silane and nitrogen oxide for oxide deposition, etc.). Flow rates are adjusted if necessary and the pump valved down to give the desired operating pressure. The RF power is then reestablished and deposition allowed to take place for the desired length of time. Several methods exist in the art for in situ monitoring of the film thickness which can be used if very accurate coatings are desired. It has been found sufficient for the embodiment illustrated in FIG. 1 for passivating coatings simply to time the deposition as it is quite reproducible from run to run.

For the reactor illustrated in FIG. 1, the following operating conditions have been found to give good depositions for silicon nitride. In an illustrative example, the nitrogen flow rate measured at 10 PSIG was 270 cm/min. The silane (diluted to 5 per cent in argon), measured at 10 PSIG had a flow rate of 24 cm/min. At a pressure of 200 microns at a temperature of 200 C and an RF power of about 10 watts-35 to 40 volts at 14 MHz, the deposition rate measured on the order of A per minute. There was no difference in uniformity detectable from slice to slice for slices on the same radius, the thickness and refractive index being measured by ellipsometry. There was less than 10 per cent change in uniformity on slices on different radii. The deposition typically lasted on the order of 100 minutes.

Using the same reactor, deposition of other compounds was affected by changing the reactant gases. Deposition parameters for coating silane oxide deposited from silane and nitrious oxide and amorphous silicon deposited only from silane are summarized in Table I TABLE I COMPOUNDS DEPOSITED Reactive Silane Amorphous Gases Oxide Silicon SiH (5% in Argon) 37 cm"' 37 cm""" N,() 270 cm"" Pressure 200 microns 200 microns Temp. 200 C 200 C RF 55 V. 22 V. Dep. Rate 85 A/min l6 A/min.

It is to be recognized that the above parameters are exemplary only and may be varied considerably without departing from the scope or spirit of the invention.

It has been observed that goo-d nitride deposition coatings are enchanced by fairly high nitrogen to silane ratio, particularly on the order of about 200 to l Oxide coatings, on the other hand, work well at much smaller ratios, typically around 50 to l. The RF voltage values depend somewhat on reactor geometery, gas flow, pressure, and etc. It is noted that RF plasma deposition process occurs in the region of the normal glow discharge. It is characteristic of this type of discharge that as the power is increased, the glow area increases until the plates are uniformly covered. This is a reasonably good criterion for determining when sufficient RF power is being coupled into the gas. Another criterion that has been found to be useful is to determine the RF voltage at the plate which corresponds to the maintenance potential V,,,, that is the potential just before the discharge extinguishes. It is then possible to determine some multiple of this value for stable operation. It has been discovered that values between 1.2 V,, and 2 V, are satisfactory.

The silicon nitride coating prepared in accordance with the present invention provide films that are tough, adherent, chemically inert, and which have considerable application for protective insulating layers or passivations. The films are uniform and pin-hole free. They also provide excellent surface conformity and step coverage if prepared at about 200 C. Nitride films prepared from nitrogen and silane with a ratio of 200 to I can be etched readily in a variety of different etchants. For example, in common oxide etch at 35 C these films etch at approximately 20 to 30 A per second.

An additional feature of the present invention to be noted is the fact that the electrodes 14 and 22 are formed within the chamber 18. This is to be distinguished from conventional plasma deposition reactors wherein it has been thought that the electrodes must be positioned external to the reaction chamber in order to prevent contamination. Applicant has discovered that at the temperature at which most depositions are carried out, this is, temperatures below 500 C the contamination from, for example, stainless steel electrodes is entirely negligible. In addition, the low power requirement of capacitively coupled RF systems (a few watts at RF voltages below 100 volts) further reduces the contamination below any detectible level.

With reference to FIG. 3, there is illustrated a flow diagram of the reactant gases that may be utilized, by way of example, for deposition of nitride coating. A source of nitrogen is illustrated at 50 and a source of silane is illustrated at 52. The nitrogen is metered through a flow meter 52, vacuum pressure gauge 54, leak valve 56, and toggle valve 58 to the mixer 60. The silane is also metered through a flow meter 62, vacuum pressure gauge 64, leak valve 66 and toggle valve 68 to the mixer 60. F rom the mixer 60, the gases flow to the reactor 70 through needle valve 72 and toggle valve 74. The nitrogen may also be fed directly into the reactor 70 by activation of the toggle valve 76 for predeposition cleaning processes. With reference to FIG. 1, the flow of the gas flowing from the mixer 60 and to the reactor 70 may more clearly be seen. The reactant gases from the mixer 60, FIG. 3, flow into the mixing chamber 40 via the tube 42. The reactant gases flow in the pattern indicated by the arrows 80 through the space intermediate the bottom plate 12 and the bottom of the electrode 22. The gases flow around the edge 28 of the electrode 22 and radially flow in a laminar stream over the slices 26 toward the aperture 36 in the center of the electrode 22. The gases are exhausted through this aperture and the sheath 34 via the vacuum means 82.

It will be noted that the cylindrical reactor of the present invention by which laminar radial flow of the reactant gases is achieved has a distinct advantage over the linear reactor in which uniformity of deposition is degraded due to depletion of silane as a function of distance. In the present situation, as illustrated in FIG. 1, it may be seen that the mass flow of gas per unit area of slice increases as the reactant gases flow toward the center 36, since the gases flow over a decreasing area. This substantially enchances uniformity of deposition. Further utilizing the radial flow pattern of the present invention, the length over which it is necessary to obtain uniformity is decreased, being only two inches for a single radius of 2 inch slices and 4 inches for two radii of 2 inch slices. This enables operation at higher pressures, substantially increasing the deposition rate as compared with the linear reactor systems.

As may be seen, applicant has advantageously provided a production quantity deposition reactor system which produces laminar radial flow of reactant gases, and provides the advantage of increased deposition rates and increased uniformity of deposition while simultaneously forming coatings on a plurality of substrates.

While illustrative embodiments of the invention have been described for forming, in particular nitride, silicon oxide and amorphous silicon coatings on silicon substrates, it will be apparent to one skilled in the art that the radial flow reactor of the present invention may be utilized for deposition of a variety of materials including, but not limited to nitrides, oxides, oxinitrides, carbides and amorphous silicon. Such coatings are useful for a variety of applications including gate dielectrics, multilevel interconnect systems, see-through photo masks, scratch resistant coatings, thin and thick film, light guides, etch masks and passivation. Various modifications may be made to the construction of the reactor of the present invention and method of deposition without departing from the scope or spirit of the present invention.

What is claimed is:

l. Coating apparatus comprising:

a. an evacuable chamber;

b. a horizontally disposed, imperforate, annular support located within said chamber and having a peripheral portion adapted to hold at least one substrate;

c. exhaust means communicating with the centrally disposed aperture of said annular support; and

d. coating vapor feed means concentric with and isolated from said exhaust means;

e. whereby said vapor feed is induced to'flow uniformly across the edge surface of the support and radially inwardly across the at least one said substrate and out through the exhaust.

2. Apparatus for coating a substrate comprising:

a. an evacuable chamber;

b. an annular support located in said chamber and having an outer region for holding said substrate and a central region defining an aperture therethrough;

c. means for generating a radio frequency discharge within said chamber adjacent said substrate whereby to form a glow discharge plasma from reactant gases within said chamber;

d. means for establishing a radial flow, from said outer region of said support toward and out of said aperture, of said reactant gases suitable for forming said plasma and coating on said substrate; and

e. means in communication with said aperture for exhausting said gases.

3. Apparatus as set forth in claim 2 including means for heating said substrate to a preselected temperature.

4. Apparatus as set forth in claim 2 wherein said means for generating a glow discharge includes substantially parallel electrodes within said chamber.

5. Apparatus as set forth in claim 4 wherein one of I said electrodes defines said support.

6. Apparatus for uniformly coating a plurality of discrete substrates with inorganic material by low temperature plasma deposition comprising:

a. an evacuable chamber;

b. a first sheath extending through the bottom of said chamber in sealing relationship;

c. a second sheath coaxially enclosed by said first sheath and extending beyond both ends thereof defining a mixing chamber in the space between said first and second sheaths which mixing chamber is in communication with the interior of said evacuable chamber, the end of said second sheath within said evacuable chamber spaced from the bottom thereof terminating in sealing relationship with the boundary of an aperture through the central portion of a support, the outer portion of which holds said plurality of substrates, the outer boundary of said support being spaced from the walls of said evacuable chamber, the other end of said second sheath disposed for connection to vacuum means, one end of said first sheath terminating at the interior boundary of the bottom of said evacuable chamber and the other end terminating in a sealing relationship between said first and second sheaths;

d. means in communication with said mixing chamber for establishing laminar flow of reactant gases over said support and plurality of substrates radially toward the center aperture of said support;

e. means for exhausting said gases through said aperture; and t f. means for generating a radio frequency glow discharge adjacent said substrates.

7. Apparatus as set forth in claim 6 wherein said support defines one electrode for establishing said glow discharge.

8. Apparatus as set forth in claim 6 including means for heating said substrate to a predetermined temperature.

9. Apparatus as set forth in claim 8 wherein said predetermined temperature is within a range from 200 to 300 C. 

1. Coating apparatus comprising: a. an evacuable chamber; b. a horizontally disposed, imperforate, annular support located within said chamber and having a peripheral portion adapted to hold at least one substrate; c. exhaust means communicating with the centrally disposed aperture of said annular support; and d. coating vapor feed means concentric with and isolated from said exhaust means; e. whereby said vapor feed is induced to flow uniformly across the edge surface of the support and radially inwardly across the at least one said substrate and out through the exhaust.
 2. Apparatus for coating a substrate comprising: a. an evacuable chamber; b. an annular support located in said chamber and having an outer region for holding said substrate and a central region defining an aperture therethrough; c. means for generating a radio frequency discharge within said chamber adjacent said substrate whereby to form a glow discharge plasma from reactant gases within said chamber; d. means for establishing a radial flow, from said outer region of said support toward and out of said aperture, of said reactant gases suitable for forming said plasma and coating on said substrate; and e. means in communication with said aperture for exhausting said gases.
 3. Apparatus as set forth in claim 2 including means for heating said substrate to a preselected temperature.
 4. Apparatus as set forth in claim 2 wherein said means for generating a glow discharge includes substantially parallel electrodes within said chamber.
 5. Apparatus as set forth in claim 4 wherein one of said electrodes defines said support.
 6. Apparatus for uniformly coating a plurality of discrete substrates with inorganic material by low temperature plasma deposition comprising: a. an evacuable chamber; b. a first sheath extending through the bottom of said chamber in sealing relationship; c. a second sheath coaxially enclosed by said first sheath and extending beyond both ends thereof defining a mixing chamber in the space between said first and second sheaths which mixing chamber is in communication with the interior of said evacuable chamber, the end of said second sheath within said evacuable chamber spaced from the bottom thereof terminating in sealing relationship with the boundary of an aperture through the central portion of a support, the outer portion of which holds said plurality of substrates, the outer boundary of said support being spaced from the walls of said evacuable chamber, the other end of said second sheath disposed for connection to vacuum means, one end of said first sheath terminating at the interior boundary of the bottom of said evacuable chamber and the other end terminating in a sealing relationship between said first and second sheaths; d. means in communication with said mixing chamber for establIshing laminar flow of reactant gases over said support and plurality of substrates radially toward the center aperture of said support; e. means for exhausting said gases through said aperture; and f. means for generating a radio frequency glow discharge adjacent said substrates.
 7. Apparatus as set forth in claim 6 wherein said support defines one electrode for establishing said glow discharge.
 8. Apparatus as set forth in claim 6 including means for heating said substrate to a predetermined temperature.
 9. Apparatus as set forth in claim 8 wherein said predetermined temperature is within a range from 200* to 300* C. 